Pair of heathkit A-7 amps need some advice

mbates14

Well-Known Member
I have a pair of these amplifiers, one of them I obtained much later than the other.

I have no idea which revision they are, but one of them have the metal 12A6 tubes, and the other has the 12V6 glass tubes. Neither of them have the pre-amp tube installed, and have a little metal plug into the tube hole.

So, I would like to rebuild these guys to match. so both have 12V6 tubes, and I have read that the transformers are a weak point here. So I ordered a small little 12v transformer I can stuff under the chassis to help offload some of the filament current.

Also, I would like to populate that pre-amp tube for a magnetic RIAA phono pre-amp, So I looked at the Heathkit schematics, but again there are multiple revisions and then I dont know what the pre-amp actually is. whether its phono or not, or for what cartridge.

any ideas? thanks!
 
The 12A6 and 12V6 don't have the same electrical characteristics, unfortunately. They are sorta close, but not exact, so depending on how precise the Heathkit engineers were, you may have different impedance output transformers in the two units. I'd start by checking the markings on the output transformers on the two units. If they do have the same part number, then you should be able to convert the one unit to 12V6 outputs.

The other difference I'm noting between the A7-b and the A7-e (these are the only two schematics for the A7 I could find) is the "e" used UL connected output stage while the "b" used pentode connected output stage (both of these schematic versions used 12A6 tubes). Perhaps you could also look at the chassis underside and determine how many wires the primaries of each output transformer has. 5 wires on the primary = UL capable, 3 wires on the primary = pentode.

Also the "e" has one 16 ohm secondary winding with taps at 4 and 8 ohms (thus four wires coming out of the secondary), while the "b" has one secondary winding with one tap (thus three wires coming out of the secondary). The schematic I have doesn't label what impedance the secondary is on the "b". So maybe you could also look at the secondary on both units and determine how many wires there are on each.

Ultimately and regardless of what you find, the next thing I'd do after that is measure these output transformers to determine the primary impedance of each of them. That will ultimately tell you if they are compatible or not.

One more thing..the 12V6 is a 12 volt heater version of the 6V6, and the 6V6 is still popular today and is made by numerous manufactures in new production. So if things work out, you could convert the output stages to 6V6, and power the heaters from a separate 6V filament transformer.
 
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F!!! they are different.

12v6 = 51-18
12a6 = 51-21

And I just noticed that the power tranny on one is different than the other. looks like a replacement.

So they are not a match. Ugh. May just dump them online or something, I dunno.
 
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the 51-21 amp has the UL taps, as well as the 12A6s.

the 51-18 is a standard push pull, has a Grand Transformers power tranny, and the 12V6s. I cant find a part # anywhere on the transformer itself, just the green sticker that identifies itself as a grand transformers made product with no specs.

So what would be the next recommendation? pull the iron and replace it with ones that match? maybe sell the old iron to recover costs?

or are they close enough to match? just curious.
 
Measure them to determine their primary impedance. Who knows they may be close. Also are the secondaries the same? (Same number of wires).
 
Yes they are. both have 4/8/16. One has UL wires, and one does not.

also the EIA codes that appear below are different as well.

What do I measure them with? Only thing I have handy is a fluke DMM and a DC power supply.
 
Do you have a variac (variable autoformer) or even another small transformer that puts out anywhere from say 25V to 100V AC on its secondary? If you do, hook it up to the Heathkit output transformer primary (end to end). Then put the Meter between the 16 ohm secondary and Com and measure the voltage. Then pull the meter off and measure the voltage delivered at the primary.

Then do the following math:

(Vpri/Vsec) * (Vpri/Vsec) * 16 ohms

This will be the impedance of the primary.

Take the power tubes out first.
 
I have a couple of 24VAC AC transformers for furnaces, thats it.

little off topic: is there a way to use that math thing above to figure out the secondaries of an output transformer? I have another amp but the secondaries are unknown. (outside of ground)

I will have to try your formula and see what I get.
 
The furnace transformer will work just fine.

To your other question, if you don't know either the primary or secondary impedances it is a little harder but you can still make some educated guesses based on type of output tube used and amp it's in.

But I think I would look at the voltage factor between the secondary taps. By driving the primary with some known constant AC voltage, the voltage on the secondary taps will differ by a constant factor. For example between Com and 4 vs 8 vs 16 ohm taps, voltage will differ by a factor of square root of 2, going consecutively from 4 to 8 to 16 ohms.

If you have oddball taps such as 3 ohms and 10 ohms let's say, then the voltage factor between those two taps will be

Square root of 10 / square root of 3
 
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The furnace transformer will work just fine.

To your other question, if you don't know either the primary or secondary impedances it is a little harder but you can still make some educated guesses based on type of output tube used and amp it's in.

But I think what I would do is to look at the voltage factor between the secondary taps. By driving the primary with some known constant AC voltage, the voltage on tje secondary taps will differ by a constant fraction. For example between Com and 4 vs 8 vs 16 ohm taps, vcoltage will differ by square root of 2 going consecutively from 4 to 8 to 16 ohms.

If you have oddball taps such as 3 ohms and 10 ohms lets say, then the voltage factor between those two taps will be

Square root of 10 / square root of 3

Math goes way over my head except for the basics of ohms law anyway. All I need to know is if the number at 16 is gonna be bigger than 4, etc.

But, the amp is a conn organ amp but starting to get sidetracked.
 
All I need to know is if the number at 16 is gonna be bigger than 4, etc.
Yes the voltage will always increase going from a smaller ohm tap to a larger ohm tap. It will increase by the amount discussed above.
 
By the way it could be possible the output transformers on your A7 amps are essentially identical except for the one having the UL taps. That would be an ideal scenario.
 
So, in order to keep the sound identical between the amps/channels, I would have to undo the UL taps, right?

I want to rebuild the amps to match as sonically as possible if I can. that means equivalent circuit.
 
I have had four of these, current pair are A7-Cs I think. Power transformers are different (Stancor and Grand), but Heath may have used Grand Transformer too - it was close by.

12V6s may not be original - I believe they were used only in the last of the series (Ultralinear A7-D and E). Used a higher cathode resistor as I recall. The phono preamp used 12SL7 in the E version, 12SJ7 or 12SG7 in the C version. Schematics and Heath ads on my page: http://www.audiophool.com/Audio.html. I'd use the later preamp version. And might as well follow the E version schematic since you don't have the tubes anyway.
 
So, in order to keep the sound identical between the amps/channels, I would have to undo the UL taps, right?
Correct. But it will all hinge on the primary impedance being the same or close to the same between the two units.
 
I have had four of these, current pair are A7-Cs I think. Power transformers are different (Stancor and Grand), but Heath may have used Grand Transformer too - it was close by.

12V6s may not be original - I believe they were used only in the last of the series (Ultralinear A7-D and E). Used a higher cathode resistor as I recall. The phono preamp used 12SL7 in the E version, 12SJ7 or 12SG7 in the C version. Schematics and Heath ads on my page: http://www.audiophool.com/Audio.html. I'd use the later preamp version. And might as well follow the E version schematic since you don't have the tubes anyway.

Well the tubes are mixed/matched in the output stage and since I want to have them all glass 12V6s, I need to know what that resistor is. the E schematic is still showing A6s.

Also the phono stage, I was googling and saw the famous "RCA" schematic that uses 7025/12AX7 tubes, and thought maybe building that around the 2C52. But I dunno if the heathkit version is better/worse than the RCA version. Thoughts on that?
 
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12V6s may not be original - I believe they were used only in the last of the series (Ultralinear A7-D and E).
Tom, do you recall which output transformer version was used with 12V6 tubes?
I have 2 units with the same PTs and OTs and the tubes are different between them.
OTs are 51-21. One amp has a single metal 12A6 (the second output tube is missing) and another amp has 12V6 pair. Both amps are UL wired with multitap output and have a phono tube. Thanks
 
I
Yes they are. both have 4/8/16. One has UL wires, and one does not.

also the EIA codes that appear below are different as well.

What do I measure them with? Only thing I have handy is a fluke DMM and a DC power supply.

Different power transformers and different OPTs? I think you will save yourself a lot of time, work and head scratching if you sell them as separates and buy a pair of matching amplifiers that need restoration (or a stereo amp). There's a risk here that no matter what you do - aside from obtaining matching iron - they won't sound the same.

Jack
 
Jack, unfortunately, this is a 5-year-old thread. I have 2 matching units but the info is confusing and just wanted to know which OT numbers were used with 12V6 tubes. Hopefully, Tom (or someone else) has this info
 
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